1. Field of the Invention
The present invention relates generally to a method of diagnosing an exhaust gas recirculation control system of an internal combustion engine primarily for a motor vehicle. More particularly, the invention is concerned with an exhaust gas recirculation control system diagnosis method capable of positively avoiding erroneous decision in the diagnosis upon occurrence of abnormality such as freezing of moisture in a pressure sensor means employed in the exhaust gas recirculation control system. The invention is also concerned with an apparatus for carrying out the method.
2. Description of the Related Art
In the internal combustion engine (hereinafter also referred to simply as the engine) for a motor vehicle, there is generally adopted an exhaust gas recirculation control system which includes an exhaust gas recirculation passage extending from an exhaust gas passage to an intake air passage for recirculating a part of the exhaust gas discharged from the engine to the intake air passage so that a maximum combustion temperature of a fuel is suppressed to such extent that NO.sub.x produced by the combustion and contained in the engine exhaust gas can be reduced. In that case, recirculation of the exhaust gas to the intake system is not constantly effected but the timing for effecting the exhaust gas recirculation as well as the amount of recirculation is controlled in dependence on the running or operation states of the engine by controlling correspondingly the actuation of an exhaust gas recirculation control valve installed in the exhaust gas recirculation passage. By way of example, in a low load range of the engine where combustion of the fuel is rather at a low level and generation of NO.sub.x is relatively insignificant, recirculation of the exhaust gas is interrupted. Further, it is generally practiced to control or limit the amount of recirculation of the exhaust gas so that content of the exhaust gas contained in the intake air remains substantially constant at a predetermined ratio.
In such exhaust gas recirculation control system, erroneous operation or occurrence of a fault in the exhaust gas recirculation control valve, a modulator for controlling the valve and other components of the system will provide obstacle to effectuation of the exhaust gas recirculation as desired. For coping with this inconvenience, there has been proposed an approach for diagnosing the exhaust gas recirculation control valve as to occurrence of some fault by detecting directly the actuated position of the recirculation control valve. This approach is however disadvantageous in that the exhaust gas recirculation control system is complicate in the structure and expensive, because there is required not only provision of a specific detector for detecting the actuated position of the exhaust gas recirculation control valve but also a processing apparatus for processing the information available from the output of the detector.
As a method of diagnosing the exhaust gas recirculation control system which can evade the aforementioned inconveniences, there may be mentioned a diagnosis method or system disclosed in Japanese Unexamined Patent Application Publication No. 11127/1988 (JP-A- 63-11127), according to which an exhaust gas recirculation control valve installed in an exhaust gas recirculation passage extending from an exhaust gas passage to an intake passage is temporarily manipulated (i.e., opened or closed) during recirculation of the exhaust gas to the intake passage on the condition that the engine load is stable, and a change in the suction pressure brought about by the manipulation of the control valve is detected, whereon decision is made as to whether the change in the suction pressure lies within a predetermined range. On the basis of the result of this decision, it is then determined if a fault occurs in the exhaust gas recirculation system.
The principle underlying this diagnosis method is based on the fact that actuation of the exhaust gas recirculation control valve is accompanied with a corresponding change in the suction pressure. Starting from this fact, it is contemplated to detect a fault occurring in the exhaust gas recirculation control valve and associated devices by checking the change of the suction pressure which is detected when a negative pressure playing a role in the actuation of the exhaust gas recirculation control valve is varied. To this end, it is necessary to apply directly the pressure prevailing within the exhaust gas passage or alternatively the suction pressure to the pressure sensor by providing a pressure introducing pipe. In this conjunction, it is however noted that moisture contained in the exhaust gas tends to be condensed as dew on the inner wall of the pressure introducing pipe or on the pressure sensor itself when the temperature of the exhaust gas is low. As a consequence, when the ambient temperature is below the freezing point, the condensate dew is frozen to decrease the flow section area of the pressure passage or the pressure sensor itself is frozen. In such situation, the change in the pressure of concern can no more be detected with any reliability. It goes without saying that diagnosis of the exhaust gas recirculation system in the state mentioned above will result in that the system suffers abnormality even when it operates normally, which leads to unnecessary generation of an abnormality alarm to the discomfort of the driver and/or wasteful exchange of components of the exhaust gas recirculation control system which suffer no fault.
As will be appreciated from the above description, in the known fault diagnosis method for the exhaust gas recirculation control system, no consideration is paid to dew condensation and freezing of moisture contained in the exhaust gas in the pressure introducing passage or on the pressure sensor itself. Thus, the known diagnosis method suffers a problem that the exhaust gas recirculation system which is operating normally may erroneously be decided to be abnormal due to inoperativeness of the pressure sensor.